Over the past three decades, considerable emphasis has been placed on the development of very thin polymeric films. For the most part, these thin films have been used as membranes, to effect chemical separations. Such processes as reverse osmosis for water desalination, gas separations as used in the purification of natural gas and in the production of oxygen-enriched air, and pervaporation which is used to break commercially important azeotropes, all rely on the availability of very thin, highly selective membranes. These and other processes are described in Volume VII of the "Techniques of Chemistry" series entitled "Membranes in Separations" (1975) by Hwang and Kammermeyer. In all membrane separation processes, the transmembrane flux is a key criterion in determining the cost of the process. High flux is generally associated with thin membranes, in keeping with Fick's first law, and considerable research and development has been expended toward making very thin, yet still highly selective membranes. "Selectivity" refers to the ability of the membrane to pass one species in a mixture while retaining other species. High selectivity is essential to effecting clean separations. The first technical breakthrough was the reverse osmosis membrane invented by Loeb and Sourirajan and disclosed in U.S. Pat. No. 3,133,132. Numerous types of membranes have been made since then using the Loeb-Sourirajan technique. See, for example, Kesting, 50 Pure & Appl. Chem. 633 (1978), who discloses asymmetric (skinned) cellulosic membranes, and Broens et al., 32 Desalination 33 (1980), who disclose similar membranes of cellulose acetate, polysulfone, polyacrylonitrile, and polydimethylphenyleneoxide.
The second breakthrough in making thin, selective membranes was due primarily to Cadotte. Cadotte borrowed from the teachings of Morgan, who first described in detail "interfacial polymerization." Interfacial polymerization (IP) is a process in which a very thin film (or membrane) can be made by reacting two monomers at the interface between two immiscible solutions. It is best described by example. "Nylons" are a class of polymers referred to as polyamides. They are made, for example, by reacting a diacid chloride, such as adipoyl chloride, with a diamine, such as hexamethylene diamine. That reaction can be carried out homogeneously in a solution to produce the polymer in resin form. However, it can also be carried out at an interface by dissolving the diamine in water and floating a hexane solution of the diacid chloride on top of the water phase. The diamine reacts with the diacid chloride at the interface between these two immiscible solvents, forming a polyamide film at the interface which is rather impermeable to the reactants. Thus, once the film forms, the reaction slows down drastically, so that the film remains very thin. In fact, if the film is removed from the interface by mechanical means, fresh film forms at the interface, because the reactants are so highly reactive with one another.
Cadotte used such knowledge of interfacial polymerization techniques to produce extremely thin, supported films such as are disclosed in U.S. Pat. No. 4,277,344. As a modification of the two immiscible liquid phases, he dissolved one reactant in a solvent and then used that solution to fill the pores of a microporous substrate membrane. He then exposed that wet membrane to a second, immiscible solvent containing the other reactant. An interfacially polymerized, very thin film formed at the surface of the microporous substrate, which then served as a support for the very thin film. Numerous adaptations of the Cadotte technology have been made using essentially the same IP method.
Morgan, in Volume 20 of the "Polymer Reviews" series entitled "Condensation Polymers: By Interfacial and Solution Methods" (1965), describes numerous chemistries that can be used to make polymers interfacially. Among the important chemistries are: polyamides, as already described; polyureas, polyurethanes, polysulfonamides, and polyesters; several other less important classes are also described. Morgan and others have also described the factors important to making continuous, thin interfacial films: temperature, the nature of the solvents and co-solvents, the concentrations of the two reactants, and the reactivity of the two monomers. Id. at pages 486-509. Refinements of the art developed over the past 20 years include the use of "blocked" or protected monomers that can be later unblocked to alter the chemistry of the finished film, the use of posttreatment of the films to alter their chemistry, and the use of heteroatoms in the monomers to alter the properties of the final film or membrane. In the classical organic chemistry sense, these alterations would be referred to as changes in the chemical "functionality," i.e., changes in those groupings of atoms that cause a substance to enter into its characteristic chemical reactions with another substance.